1. Field of the Invention
The present invention relates to a single crystal garnet film which exhibits a uniaxial magnetic anisotropy perpendicular to a film plane and is suited for supporting magnetic bubbles in a magnetic bubble memory element.
2. Description of the Prior Art
As is well known, the magnetic bubble memory element attracts an attention as one of promising memory devices and many endeavors to develop more practical magnetic bubble memory are (made with activity in many spheres). Among parameters which determine a storage density, (which is the most important factor for memory performances) is a diameter (d) of a magnetic bubble. When the bubble diameter (d) is smaller than 2.5 .mu.m, the storage density or capacity will be remarkably enhanced.
In other words, in order to use the magnetic bubble memory in practical use replacing other memories such as disc memories, semiconductor memories, it is very necessary to reduce the bubble diameter as far as possible, and to increase the storage density significantly.
It is known that the magnetic garnet film with small bubbles has a serious trouble of a large temperature dependence of the bubble collapse field (Ho).
For example, in the case of a garnet film of (YSmLu).sub.3 (FeGa).sub.5 O.sub.12 supporting magnetic bubbles with a diameter of about 2 .mu.m, of which temperature coefficient of Ho at 30.degree. C. is in a range from -0.30%/.degree.C. to -0.35%/.degree.C.
On the other hand, the corresponding temperature coefficient of a bias field applied by a barium ferrite magnet, which is usually used as a bias magnet in conventional bubble devices, is -0.2%/.degree.C. Thus, there is a large difference between the garnet film of the composition mentioned above and barium ferrite magnet in respect of the temperature dependence of the required magnetic field. It is obvious that a great difference of the temperature coefficient of Ho and bias field Hb will necessarily narrow the temperature range in which the bubbles can be exist controllably. This is unfavorable to use magnetic bubbles for the memory.
By way of example, temperature characteristics of the garnet films for the magnetic bubble memory elements are described in the following references:
(1) R. M. Sandfort, et al., "Temperature variation of Magnetic Bubble garnet film parameters", AIP Conf. Proc. 18, (1) pp 237-241 (1973).
(2) G. G. Summer, et al., "Growth Reproducibility and Temperature Dependencies of the static properties of YSmLuCaFeGe Garnet" AIP Conf. Proc. 34, pp 157-159 (1976).
(3) Jerry W. Moody, et al., "Properties of Gd.sub.y Y.sub.3-y Fe.sub.5-x Ga.sub.x O.sub.12 films grown by LPE" IEEE transactions on magnetics, Vol. Mag. 9, 377 (1973).
The reference (1) discloses the temperature characteristics of garnet films for the magnetic bubble memory. However, there is no teaching in respect of improvement on the temperature characteristic of the bubble collapse field Ho of the garnet film for the small magnet bubbles.
In the reference (2), garnets of (YSmLuCa).sub.3 (FeGe).sub.5 O.sub.12 are described, which has a temperature coefficient of Ho to be -0.20%/.degree.C. However, in such a garnet composition, the temperature dependence of bubble collapse field Ho is fixed. It is impossible to control the temperature dependence of Ho to a desired value which is most suitable for the bias field.
The reference (3) discloses garnet compositions containing Gd and Ga. However, these compositions are not intended for use as the materials for the small magnetic bubble devices. Further, there is no description about the bubble collapse field Ho.